Generally, a mold used for molding of a rubber product etc. including a tire is provided with a vent hole for exhausting air from the interior of the mold to prevent an air bubble defect from developing in a molded product. The vent hole, which is a small-diameter hole formed in a mold so as to connect the inside of the mold to the outside thereof, is sometimes provided with a valve element for opening and closing an inside opening of the hole to prevent outflow of a raw material in the mold (for example, Patent Document 1).
FIG. 5 is a sectional view of a vent hole disclosed in Patent Document 1. A vent hole 101 consists of a through hole 103 penetrating a mold 102 and a central hole 105 in a cylindrical vent piece 104 inserted in an insertion portion (diameter increasing portion) near the inner end of the through hole 103. At the inner end of the vent piece 104, a disc-shaped valve element 106 is installed to open and close the central hole 105.
The vent piece 104 is fixed to the mold 102 by press fitting or staking into the insertion portion, interference fitting utilizing thermal expansion and contraction, bonding using an adhesive, or the like.
[Patent Document 1] Japanese Patent Laid-Open No. 2004-291615 (paragraph Nos. 0019 to 0021, FIG. 1)